If you grab a firearm or a melee weapon with your hands and then you leave it in the floor the item disappear and then reappear in your inventory. I had a mosin and a fire axe in my back and i grab a carbine with my hands, then i leave it and the item disappear, when i hit "tab" the carabine was in the bottom of my inventory, as a test i move far of my position (with the mosin and the carabine in my backpack) and i was able to grab the carabine of me backpack and leave it the floor, with the mosin in my back.

The same with a hammer and my fire axe